Abstract: The slug flow in marine risers is severe. In this paper, the characteristic method was used to explore this phenomenon. The severe slug flow presents different characteristics in four stages. According to the continuity equation and momentum conservation equation of gas-liquid phase in downdip pipe and riser, combined with gas compressibility, the transient flow models corresponding to four stages were established in constant temperature. And then, a quasi-equilibrium one dimensional physical model was developed for the severe slug flow in riser system. Finally, the calculation results were compared with the experimental data. It is shown that the caculated characteristic parameters (e.g. slug period and riser bottom pressure) are in good agreement with test data.